The present invention relates to apparatus for reproducing stereo sound, and particularly to apparatus for reproduction of stereo sound from a two channel stereo sound signal.
Stereo sound recording and reproduction employs stereographic projection to encode the relative position of sound sources recorded, and aims to reproduce the sound with a sense of those relative positions. A stereo system can involve two or more channels, but two channels systems dominate for audio recording. The two channels (usually known as left and right) convey information relating to the sound field in front of the listener. By far the most popular means for reproducing two channel stereo signals is to broadcast the channels via two respective, spaced apart, left and right loudspeakers.
Despite its popularity, though, there are disadvantages with this system. Most commercial two channel stereo sound recordings are mixed for optimum reproduction by loudspeakers spaced about 1.6 meters apart. In reality, this is rarely possible, especially where it is desired to reproduce stereo sound from a single unit. In any event, however a recording is mixed, the closer the loudspeakers used to transmit the left and right channels are together, the poorer the stereo effect reproduced.
Also, for optimum perception of stereo effect the listener should be located at the apex of an equilateral triangle made by the pair of loudspeakers and the listener. In reality, though, it is often inconvenient or impossible for a listener to adopt or maintain this position and, of course, it is impossible for multiple listeners to listen from the same position.
An alternative system for reproduction of a two channel stereo signal which should overcome some of the above disadvantages has been proposed. This system, which shall be referred to as the sum and difference system, is disclosed in U.S. Pat. No. 3,588,355. This document discloses a stereophonic loudspeaker system comprising two pairs of loudspeakers. Each pair is oriented with their axes at right angles to each other and substantially equidistant from the point of intersection of the axes. The speakers are so arranged that one speaker of each pair faces the listener and the other speaker has its axis substantially perpendicular to the listener. Means are provided for matrixing left and right two-channel stereo signals to provide a sum signal and a difference signal. The sum signals are applied in phase to the speakers whose axes are directed toward the listener, and the difference signals are applied to the speakers whose axes are positioned at right angles with respect to the direction of the listener, the difference signals to the two perpendicular speakers being applied 180° out of phase with each other:—As a result, a stereophonic sound effect should be produced by the system.
Whilst intended to overcome the problems associated with the use of spaced apart speakers the arrangement of U.S. Pat. No. 3,588,355 has not entered widespread use. This is thought to be because there are practical difficulties with the disclosed apparatus which result in the actual sound quality and perceived stereo effect obtained falling below what might theoretically be expected.
The present invention seeks to provide improved apparatus for the reproduction of stereo sound.
According to a first aspect of the invention there is provided apparatus for reproduction of stereo sound comprising a first transducer arranged to reproduce one of two audio signals and a second transducer arranged to reproduce the other signal, and one or two further transducers arranged to reproduce a signal comprising the difference of the two audio signals.
This arrangement combines advantages of conventional two speaker systems for reproduction of stereo sound, and the sum and difference system. Appropriately directed difference signals can be employed to enhance the perceived width and depth of the signal reproduced. This is useful where the perceived width and depth of the signal reproduced by the first and second transducers alone is limited. This is the case where two transducers are arranged close together, for example in the same housing in portable stereo reproduction equipment.
The apparatus may comprise a housing defining an at least partially enclosed space. All the transducers may be mounted in the housing, and may all communicate with the partially enclosed space; The first and second transducers are preferably arranged to direct their output in substantially parallel, spaced apart directions. The two transducers arranged to reproduce the difference of the two signals are preferably arranged to direct their output primarily in a direction generally at right angles to the direction in which the first and second transducers are arranged to primarily direct its output, and may direct their output in generally opposite directions.
According to a second aspect of the present invention there is provided apparatus for reproduction of stereo sound comprising a housing defining an at least partially enclosed space, a first transducer arranged to reproduce one, or the sum, of two audio signals and two transducers each arranged to reproduce a signal comprising the difference of the two audio signals, wherein the three transducers all communicate with the at least partially enclosed space and wherein the transducers arranged to reproduce the signal comprising the difference of the audio two signals are each arranged to direct their output primarily in a direction generally at right angles to the direction in which the first transducer is arranged to primarily direct its output.
Where the first transducer is arranged to reproduce one of two audio signals a second transducer is preferably provided to reproduce the other signal, and the first and second transducers are preferably arranged to direct their output in substantially parallel, spaced apart directions.
It is thought that the sum and difference system of stereo sound reproduction works by the broadcast audio sum signal being modified by the broadcast difference signal by varying amounts at different locations to recreate the original recorded sound field, or an approximation of it. Where first and second transducers are provided to transmit respective audio signals a similar effect to broadcast of the sum of the two signals by one or more transducers is obtained, especially if the two transducers are close together. Having all three, or four as the case may be, transducers communicate with the same partially enclosed space confers two distinct benefits. Firstly it simplifies the mechanical construction of the apparatus, particularly the housing, there being no need to construct baffles or ports to separate the transducers. For given transducers this can reduce the overall size of housing required, and/or can allow mounting of other components in the housing. Likewise this may enable a pair of speakers arranged to transmit a difference signal to be incorporated into available space in the housing of existing mono or stereo audio equipment to enable the equipment to be redesigned to reproduce a stereo or improved stereo audio signal without the need to increase the size of the housing. Secondly, it provides for increased interaction between the output of the transducers which can add increased perceived depth and width to the sound reproduced by the apparatus.
According to a third aspect of the present invention there is provided apparatus for reproduction of stereo sound comprising a housing arranged to be placed on or adjacent a generally flat surface, a first transducer arranged to reproduce one, or the sum, of two audio signals and two transducers each arranged to reproduce a signal comprising the difference of the two audio signals, wherein the transducers arranged to reproduce the signal comprising the difference of the two audio signals are each arranged to direct their output primarily in a direction generally at right angles to the direction in which the first transducer is arranged to primarily direct its output and wherein the two transducers arranged to reproduce the signal comprising the difference of the two audio signals are arranged relative to the housing such that when the housing is placed on or adjacent a generally flat surface both transducers are oriented to direct their output primarily in a direction extending towards the plane of the surface.
By directing the output of the transducers reproducing the difference signal towards a surface against which the apparatus is used any surface effect caused by the surface, and which is generally found to improve the output of the apparatus, is enhanced.
Where the first transducer is arranged to reproduce one of two audio signals a second transducer is preferably provided to reproduce the other signal, and the first and second transducers are preferably arranged to direct their output in substantially parallel, spaced apart directions.
Apparatus according to any aspect of the invention may include any or all additional features of another aspect of the invention, as appropriate.